This invention pertains to network communication systems, and more particularly, to such a system having a terminal unit with message repeating capabilities.
The piggy-backing of a communication system over an electrically conductive network used for other purposes have had much developmental work. Communication over one such network, a power distribution network, offers particular advantages to electric utilities because it provides communication to those points where the electrical power is distributed without installing a separate communication medium.
These systems have been used for a variety of purposes, including office communications, load control, alarm systems, and meter reading. However, the power distribution network provides a uniquely hostile environment for communications. Interfering phenomenon include spikes typically of several volts and amplitude with frequency content into the megahertz region, occurring several times per power line cycle; high signal attenuation due to large RF loads, particularly capacitors embedded in other equipment; dynamically variable signal attenuation due to changing loads; highly non-uniform transmission as a function of frequency; other continuous wave signals; and lack of coupling between windings of distribution transformers.
Almost all network communication systems which have been developed are directed over a pyramid network, typically from a substation to individual power delivery terminal units on a secondary feeder line. Communication takes place between the central control unit and the remote terminals. In these systems it is typical to put repeaters at transformer locations in order to overcome the typical high attenuation at such points. Further, additional repeaters are often used for long primary or secondary distribution lines. Specifically, attention has been given to avoiding oscillation, addressing repeaters, and routing messages. In all cases, the repeater has been a separate, stand-alone device uniquely dedicated to serving as a repeater. Furthermore, effort is required to determine the proper location of a repeater which typically is designed for a particular network. When that network changes, repeater locations also have to be changed.
Typical efforts which have been made in this area are described in the following U.S. patents: U.S. Pat. No. 3,693,155 issued to Crafton et al.; U.S. Pat. No. 3,911,415 issued to Whyte; U.S. Pat. No. 3,942,168 issued to Whyte; U.S. Pat. No. 3,944,723 issued to Fong; U.S. Pat. No. 3,967,264 issued to Whyte et al.; U.S. Pat No. 3,973,087 issued to Fong; U.S. Pat. No. 3,973,240 issued to Fong; U.S. Pat. No. 4,210,901 issued to Whyte et al.; U.S. Pat. No.4,250,489 issued to Dudash et al.; and U.S. Pat. No. 4,427,968 issued to York. The last patent in this list, as an example of a pyramid network system, discloses the use of repeaters which can be used to relay messages to terminal units. If the distribution network changes the repeaters can be downloaded with route information to accommodate different message routes.
Heretofore, there has thus been a need to install repeaters along a network where a central unit and a terminal unit are not in direct communication. In an environment which has a large number of terminal units there may thus be a substantial redundancy of equipment required. For example, a power distribution system used to monitor or read meters at power delivery points would have a large number of terminal units communicating with a remote central unit. Further, in an office environment where there is communication between individual terminal units, there are typically a large number of terminal units communicating over a single distribution network. Usually in such an environment, each terminal unit is able to communicate with at least one other terminal unit but not necessarily with all other terminal units. An elaborate design process is required in such instances to determine where repeater units need to be located and how they need to interface in order to provide communication between all of the terminal units. When changes take place on the network, a reanalysis of the network is necessary in order to determine where changes in repeater unit locations are required. Further, in a system in which message traffic is limited to travel between individual terminal units and a central unit, there has been little need to accommodate more than a very few message routes for each repeater. In an office communication system this is inadequate because of the multitude of possible message routes which exist.